Some changes are related to the way multinode works (after the final
bits have been settled during the PTG) and others are just bug fixes
that were not noticed before because the entire job did not work.
- switch to the orchestrate-devstack role as needed for multinode jobs;
likewise, explicitly set the execution strategy to linear;
- remove few Swift related settings (services, a configuration
variable) which are now set by default by devstack;
- increase the size of the default loop disk used by Swift
and the maximum file size, otherwise the custom-built image
will not fit;
- fix the variable which points to the sahara configuration
file so that the the value set is injected in post-config;
- rework the creation of the custom flavor, and actually
use the created flavor for the job;
- use devstack-admin credentials for now (in the future
it may be devstack, but it may require more changes);
- remove the "voting: false" attribute because
non-voting is implicit for experimental jobs.
Story: 2001686
Task: 8659
Change-Id: I51c1628f66beb827f3450f4432db8c985cd38f7f
b5c5602f5b